1. Field of the Invention
This invention relates to semiconductor wafer processing reactors and, more particularly, to an improved clamp ring for clamping a semiconductor wafer onto a pedestal used in such processing reactors.
2. Background
Semiconductor wafers are generally processed in thermal reactors in which the wafer is subjected to a number of different processing steps. In certain wafer processes, for example plasma etching and sputtering, the wafer to be processed is held down onto a domed pedestal by a clamping device such as a clamp ring.
An example of how wafers have been mounted in a plasma reactor is illustrated with reference to FIG. 1, which is an exploded sectional view of the relevant internal components of the reactor. This figure shows a semiconductor wafer 10, a circular wafer support 12 on which four (of which only two are shown) lift fingers 16 are mounted, a domed pedestal 18 and a clamp ring 24.
Before wafer processing commences, the semiconductor wafer 10 is brought laterally into the reactor by means of a robot arm (not shown). The wafer support 12 then moves up from below the wafer 10 to lift it clear of the robot arm by supporting it on the flat support faces 14 of the four lift fingers 16. The robot arm then leaves the reactor, and the support 12 moves downward to bring the wafer 10 into contact with the domed upper surface of the pedestal 18. (As is shown in this figure, the support 12 moves along a common central axis 19, indicated in broken lines, which it shares with the pedestal.) Thereafter, the clamp ring 24 moves down onto the wafer so that the wafer 10 is received in a flat faced, annular seat 26 formed in the clamp ring's underside.
The clamp ring 24 continues to move downward, forcing the wafer 10 to adopt approximately the profile of the domed surface of the pedestal 18. It is in this configuration, which is illustrated in greater detail in FIG. 2, that the plasma etching process steps occur.
Before describing FIG. 2 in detail, it should be noted that although not evident from FIGS. 1 or 2, the pedestal 18 has an outer diameter larger than that of the outer diameter of the wafer 10. Furthermore, to enable the support 12 and the fingers 16 to move past the pedestal 18, four cutouts 20 are formed in the sides of the pedestal. Typically, the pedestal 18 is made of aluminum and includes water passages for passing cooled water to cool its domed surface. The pedestal operates to cool the wafer 10 during wafer processing. It can also be electrically charged during plasma etch processes to act as a cathode so as to ensure correct wafer etching. To enhance transfer of heat from the wafer to the domed surface and improve uniformity of this transfer, gas is injected into the space between the wafer and the domed surface via a gas conduit 22, formed along the central axis 19 of the pedestal 18 to exit in the center of the dome.
FIG. 2 is an enlarged detail of the area where the clamp ring 24 engages the edge of the water 10 and illustrates the problem with the prior art clamp rings. As is apparent from this FIGURE, the flat face of seat 26 is horizontal, while the surface of the pedestal, and accordingly the wafer when it conforms to the dome of the pedestal, are curved. As a result, the clamp ring 24 contacts the wafer 10 only in a line at edge 30.
The problem with this arrangement is that the wafer's surface can be damaged by the edge 30 scraping it. Alternatively, if the wafer is much harder than the clamp ring 24, the clamp ring can be damaged by the wafer. Damage to either the wafer or the ring can lead to undesirable particle generation during the processing operations.
This problem is further exacerbated when gas is injected into the gas conduit 22 and forced into the space between the backside of the wafer 10 and the domed surface of pedestal 18. The gas, usually an inert gas such as Helium, is typically maintained at a pressure of between 4 to 18 Torr while the pressure in the interior of the reactor is at 2 to 10 milliTorr during processing. As a result of this pressure differential the wafer flexes and bows away from the pedestal 18, a configuration indicated by broken lines 10'. Not only does this flexing cause even greater pressure on the wafer's surface by the edge 30 of the clamp ring 24, but it also worsens the scraping of the surface of the wafer 10 by the edge 30.
In the past, attempts have been made to overcome this problem by very accurately machining and precision polishing the seat 26 and particularly the right-angled edge 30. This machining and polishing process requires a great deal of care by skilled operators and is therefore very costly. Unfortunately, however, even the most careful polishing and resulting deburring of the right-angled edge 30 is still not sufficient to provide a totally defect-free edge. Any imperfections in this edge 30 act as concentration points for the stress generated by the clamp ring as it holds the wafer down onto the pedestal. These stress concentration points cause damage which can be particularly troublesome as a silicon wafer behaves much like a piece of glass; i.e., a small chip or score mark on the wafer's surface may propagate from the point of stress and shatter the wafer. Additionally, such scoring and marking of the wafer's surface disrupts its planarity. Thus, subsequent processing, for example with a material such as tungsten-CVD (which is difficult to bond to a wafer's surface even under ideal conditions) may not be possible with any measure of reliability as the tungsten may not be able to adhere to the damaged portion of the wafer's surface. As a result, the tungsten may tend to lift away from the wafer's surface during further processing.
Another way of solving this problem has been suggested in U.S. patent application Ser. No. 08/037,988 filed on Mar. 24, 1993 in the names of Banholzer, et al. This application teaches the use of a clamp ring in which the face of the seat is angled upwards at an angle at least as steep as the slope of the domed surface of the pedestal.
Notwithstanding these proposed solutions, there exists a need for a clamping a semiconductor wafer onto a domed pedestal in a thermal reactor.